External bus interface

As long as the NIOS is in complete control you will not need to make it a master.

So if you are simply reading and writing to this device from the NIOS then you want to use it as a slave. With that said the error messege for that one just means that a signal name doesn't follow the normal conventions used (I'm guess it's the underscore but don't quote me on that). Is "e_signal" you're own name? If so just change it to something like esignal or signalEn (depending on what it does).

Like I said I don't know if that's the cause or not (because it creates signals like read_n for example by default).

Try turning off the Import button, then click on generic port list then modify that to what you want.